The hypothetical spherical shell of debris 50,000-100,000 AU in radius from which some comets are thought to emanate. The cloud is believed to be made up of material ejected out of the inner solar system by encounters with Uranus and Neptune, but which remains gravitational bound to the Sun. At their extreme distance ( parsec), the estimated comets in the cloud are so loosely bound that they are susceptible to gravitational perturbations from passing stars which can send them back into the inner solar system. The Oort cloud is believed to be the source of long-period comets (periods y), and may also be the source of short-period comets (periods y).

This idea is based on two observational facts: (1) the inclinations of long period comets show a random distribution, (2) a histogram plot of the aphelia of observed long-period comets shows that there are two peaks, one near 10,000 AU and another near 50,000 AU. The latter may correspond to the Oort cloud. However, since comets have nearly parabolic orbits the location and even the existence of these peaks is highly uncertain.

Kuiper Belt